Signaling system



Oct. 25, 1949. R. c. FERRAR ET AL SIGNALING SYSTEM Filed Sept. 18, 1946 E & rzm Y w m Z M V N! e m CM/M fd r/ e 0/ Z M "A F n Nut-4.224

Patented Oct. 25, 1949 UNITED STATES ATENT OFFICE SIGNALING SYSTEM Robert C. Ferrar, New Providence, and Gerald Menhennett, Red Bank, N. 3., assignors to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application September 18, 1946, Serial No. 697,670

9 Claims. 1

This invention relates to systems for selectively calling stations in a communication system and for signalling the call. I

An object of the invention is to call a particular desired station, place it in operating condition and to signal the operator thereof.

Another object is to provide a signalling system capable of signalling the operator when a call is coming in, and if not received by an operator, to leave an indication that a call has been attempted.

In our copending application for Selective calling system, Serial No. 695,544 filed September 7, 1946 (Ferrar et a1. 4-1) assigned to Federal Telephone and Radio Corporation we disclose and claim a selector call system wherein each remote or field station is provided with a number of vibratory elements, each adapted to be set into vibration by a particular calling frequency. By

transmitting from the central station successive- 1y those frequencies which are required to operate the vibratory elements at a given field sta tion, suitable charging circuits associated with the vibrating elements are caused to operate a voltage-operated device to render the receiver at the called station operative. By this system, single, selected groups or all of the field stations can be called as may be desired.

In the present invention we provide signalling means, either audible or visual or both, to let the operator know that a call is being made, this being desirable since the receiver is provided with earphones instead of a loudspeaker. These signals are produced by a ringing signal after a control circuit has conditioned the receiver in response to a calling signal. The system includes a second control circuit which operates in coordination with the first or main control circuit. The two control circuits control the signalling devices so that one or both operate while the call is attempted and thus maintains at least one in signalling condition until the operator lifts the earphones.

The foregoing and other objects and features of our invention will be more thoroughly understood from the following detailed description and the accompanying drawing which shows a system embodying our invention.

In the drawings the figure shows an arrangement according to our invention, adapted to be incorporated into a receiver of the frequency modulation type.

In practice there will be one of the systems shown in the drawing, for example, incorporated into each receiver which is to be called from the central or calling station. In the figure, the tube l represents an audiofrequency power amplifier which may be any desired type of power amplifier; and as the particular type is unimportant, all of its electrodes are not shown, but only the output electrode or anode 2. Power amplifier l is part of the receiver and it is coupled through an output transformer 3 to earphones 4.

Upon being called from the central station, this audio-amplifier is adapted to be made operative by the system shown connected with it. The system operates on audio-frequency and its input is connected over a lead 5 from the output of the usual discriminator of a frequency modulation receiver, through a coupling condenser 6 to the control grid 7 of an audio-frequency amplifier tube 8 of the triode type. The anode 9 has its output connected to a coil which is electromagnetically related with a number of tuned reeds of which there are shown three, these being numbered ll, l2 and I3. The reeds are each tuned to a different natural frequency in the audio-frequency range so that when the tuned frequency is sent through coil [0, the corresponding reed will vibrate against the respective reed contacts l4, l and 16.

Reed I I has connected in series between it and ground a high resistance ll; reed I2 is connected to ground through a series arranged resistor 18 and condenser l9; and'reed I 3 is connected directly to ground. Contacts l5 and I6 are connected together and are connected to ground through two parallel paths, one of which comprises the series-connected condenser 20 and resistor 2!, and the other of which comprises the series arranged resistor 22 and condenser 23.

For the purpose of controlling the audio power amplifier I, in accordance with called signals, there is provided a thyratron tube 24, the control grid 25 of which is connected to points 26 and 21 which are the points between condenser 20 and resistor 2| and between resistor 22 and condenser 23, respectively. The output circuit of the thyratron connected to its anode 28 passes through a relay coil 29 having a pair of armatures 3i! and 3| adapted to make and break contact with the respective relay contacts 32 and 33. Contact 32 has connected to it a voltage source 34 for supply of negative bias for tube 24. The armature 3B is connected over lead 35 to the grid 25 of the thyratron, so that when the armature closes on its contact 32, the negative bias is connected to the thyratron grid to shut off the thyratron. The armature contact 33 is connected over lead 36 to the lower end of a resistor 31 which is connected in series with relay coil 29.

' The relay armature 3| is connected over a lead 38 to the lower side of the primary winding of output transformer 3, so that when armature 3| connects with its contact 33, the B voltage which is supplying the thyratron is connected directly to the anode 2 of the audio power amplifier of the receiver, thereby making the power amplifier and hence the'receiver, operative.

If desired contact 3 3 may not be connected to the primary of transformer 3, the latter in that case being connected directly to the power source. With such a connection the receiver would. always be operative upon reception of the carrier to which it is tuned, the control circuit serving only as a control for the signalling: equipment.

To describe the operation of the calling system, let it be assumed that reed II has a natural frequency of vibration at a frequency X somewhere in the low audio range, and that reeds l2 and I3 have somewhat diiferent natural frequencies Y and Z, also in the low audio frequency range. The operator at the central station will have a number of calling keys, or dialv with a plurality of'keying positions, each of which will frequency modulate his transmitting carrier frequency by a sustained note somewhere preferably in thelow audio range. To call the particular station represented in the drawing, which has the particular reeds with frequencies X, Y and Z, the operator will first dial or push his key for the X audio frequency; and when the receiver picks up the signal frequency modulated by the X signal, the discriminator will separate out the X frequency in the usual way and send it to audio amplifier 8'. Since the reed operating coil ill will carry this note, it will vibrate the reed H without vibrating either of reeds I 2 or l3 since the latter two reeds are not naturally resonant at this frequency X. While reed H is vibrating, it will rapidly make and break contact with contactor l4 and the B voltage connected to the lower end of coil Ill will send a charging current through resistor l8" and condenser Hi to charge up the condenser 19, in a brieftime; This charge on the condenser will leak off in a. short period of. time of say two or three seconds through resistors I8 and IT which are in. series. across the condenser, the actual time depending on the time, constant of the circuit.

Before the charge leaks off'condenser [9, however,v the operator will dial or push the next call signal Y, which will cause reed I2 to vibrate against its contactor l5. at. the low audio rate, and this will cause a current. from the charged condenser l-il to pass through condenser 20, thereby charging up condenser 28, the charging circuit for condenser 20 being through the. parallel-arranged elements: 2i and 23 and the conductive grid-cathode circuit of the thyratron tube 24. The charge on condenser 29' will gradually leak oif through the resistance. connected across it, but before this happens, the operator at the calling station will; dial or pushhis Z button to vibrate reed l3 against its contact I6, thereby connecting the positively polarized upper end of the condenser 20' to ground intermittently, and

causing the upper end; of condenser 23, and; the

thyratron grid connected to it, tobe driven negativev with respect to ground and the thyratron cathode. Since the charging path for condenser 23', through condenser 2c and the contacts of conduction which is otherwise occurring within the thyratron tube will cease.

The cessation of conduction in the thyratron causes the D.-C. current in the relay coil 29 to drop to zero, so that the armatures 30 and 3| will drop back to their normal position of closed contact wit'h theirrespective contacts 32 and 33. The closureof these relay contacts performs two functions: The closure of armature 3| with contact 33 applies B voltage to the plate of the audio power amplifier, causing the signals at the transmitting or central station to be heard in the earphones-4. The closure of armature 30 against contact 32- applies a constant negative voltage to thethyratron. grid, thereby causing the thyratron to remain in a non-conducting state. When the carrier is-shutofi. at the time that the operator at. the central station discontinues the signal, the cessation of the carrier at the limiter grid causes thermal agitation noise output from the lilniter'stage of the receiver. This noise voltage is applied through condenser 39 to'grid 2 5 thereby trigging thyratron tube 24, and allowing the thyratron to begin to conduct again. This cond'uctionproduces current inrelay coil 29; pulling in the two armatures wand 31 again; and thereby rendering the audio power amplifier again-inopera tive.

It will be recognized that the. invention may be used to call individuallyany one of a number-0f stations, the number being dependent only on the number of reeds per station, and. the number of audio-frequency call signals per system. If; N'be the number ofreeds per second, and Tthe number of call tones available at; the controlstation, the number ofremote stations: which may be individually called is:

4 different frequencies. although reed [2' should have a natural frequency diflere'nt from either reed H or I3.

In. the. operation of systems. of thisv character it is sometimes desirable to. calla particular group of field. stations and at other times. it is desirable: tocallLall of. the fieldstationssimultaneously, To. call a particulargroup it. is only necessary to. transmit, in the. caseof three reed. signalling. circuits, one additional frequency thereby providing. in the four frequency signal. several. three. frequency signal combinati0ns.. This may also beincreased by. adding still another frequency to the combination signals Wherr all field stations arev to. be, called allof. the signal frequencies could be. transmitted but it. is preferable to provide. a much simpler signalling arrangement and this is accomplished: by: including a. fourth vibratory element- 40 for actuation: by coil Ill. in response to. a predeter.

' mined frequency at which element 40' isresonant.v

The element-40 is connected in; circuit-with the associated contact M between the'grid 25:by.con-- nection 42 and a negative voltagescurceby connection 43;

Havingdescribed the. call selector circuit. of our aforesaid copending application, we'willnom describethe-signalling equipmentand how the controls therefor coordinate with: the selector call cdircuitfor-proper operation of; the signalling sis-- ces.

' The figure of the drawing shows audible and visual indicating means for indication of a call signal and if not answered to leave an indication that a call has been attempted. The signalling portion of the circuit includes an audible signalling device 44 which may comprise a bell and the visual indicating device 45 which may comprise a light bulb. These .two devices are controlled by a relay comprising a coil 46 and three switches 41, 48 and 49. The bell 44 is connected in circuit with switch 3| through connection 50 and through switch 41 to ground. Thus when both switches 3i and 41 are closed the bell will receive current from battery B. The light bulb 45 is connected between ground and by connection 5! through switch 48 and connection 52 to battery B. The relay 46 is controlled by circuit 53 which includes a reed 54 and contact 55, connection 56, switch 3| and connection 36 to battery B. Coupled in parallel with the coil 46 is a condenser 51 p and resistor 58, the parallel circuit being connected to ground 59, whereby reed pulses are integrated for operation of the relay in response to reed vibrations.

The relay 46 is further controlled by the earphones 4. The receiver hook 60 is provided with a switch 6| which normally closes with contact 62 when the earphones are on the hook. This completes circuit connections 62 and 63 between switch 49 to battery B by way of connection 52. .i'

vibratory elements ll, l2 and I3 in the manner hereinbefore described to block thyratron 24 thereby permitting switches and 3| to close. Since the earphones 4 control the holding circuit for relay 45, the operator at the receiver must be signalled to lift the earphones from the hook 60. The closing of switch 3| completes a circuit with reed 54 and contact 55 which responds to a ringing frequency signal to energize relay coil 46 closing switches 41, 48 and 49. Closed switch 41 completes the circuit through switch 3| for bell 44, causing it to ring. Closed switch 48 completes the circuit for energization of lamp 45. Closed switch 49 completes a holding circuit through contacts 6|, 62 associated with hook 6B. When the earphones are lifted from the hook 60 the holding circuit of switch 49 is opened thereby deenergizing relay 46 resulting in the opening of switches 41, 48 and 49. Thus the indicatinge devices 44 and are both deenergized.

In case the operator is not present the bell 44 will continue to ring as long as the receiver is being called. As soon as the call is terminated by interruption of the carrier frequency the relay switches 30 and 3! are opened by the renewed operation of thyratron 24 in response to thermal noise occurring in the limiter circuit as hereinbefore described. The opening of relay switch 3|, however, does not deenergize the light bulb 45. The light bulb being controlled solely by switch 48 continues tobe energized until therelay 46 is deenergized by the breaking of the holding circuit controlled by hook 60. Thus when the operator returns and sees light bulb 45 lit he knows that an attempted call has been made. By lifting the earphones 4 the holding circuit is broken and the light bulb 45 deenergized.

While we have disclosed our invention with a particular circuit arrangement it should be understood that such arrangement is to be regarded as illustrative of the invention only and not as limiting the scope thereof.

We claim:

1. In a receiving station adapted to be called from another station, a control circuit, a call indicating circuit, means including vibratory station selector apparatus-operating responsive to a station selector signal to cause said control circuit to condition said call indicating circuit, and vibratory means responsive to a further received signal to energize said call indicating circuit.

2. In a receiver station according to claim 1 wherein the station includes a reproducing means, and said call indicating circuit includes a relay having a holding circuit controlled by said reproducing means.

3. In a receiver station according to claim 1 wherein said indicating means includes an audible indicator and a visual indicator, each operable upon reception of said further signal, and means to maintain one of said indicators operative after termination of an attempted call.

4. In a receiver station adapted to be called from another station; a first control circuit, a second control circuit conditioned only by operation of said first circuit, first and second signalling devices, the first being conditioned by operation of said first control circuit but both being controlled by energizatlon of said second control circuit, means responsive to a carrier modulated with a predetermined signal for operation of said first control circuit, means responsive to a further modulation signal to energize said second control circuit, said .first control circuit being held in operative condition so long as said carrier is received, and means to maintain said second control circuit energized until an attempthas been made to receive the call.

5. In a receiving station adapted to be called from another station, signal responsive means adapted to be energized by a predetermined signal with which a carrier is modulated, a first relay normally maintaned in open position, means to move said first relay to closed position upon energization of said signal responsive means, a second relay normally maintained in open position, means responsive to reception of a further predetermined signal after prior closure of said first relay to actuate said second relay to closed position, a signalling device operable upon closure of both relays, means for reproducing intelligence signals received by said station, means controlled by said first relay for controlling reproduction by said reproducing means, and means associated with said reproducing means to maintain said second relay closed upon closure thereof until said reproducing means is used to receive a call.

6. In a receiving station adapted to be called from another station, an electromagnet adapted to be energized by signal frequencies with which a given carrier is modulated, a plurality of vibratory elements each resonant at its respective signal frequency, a first relay normally maintained in open position, means to move said first relay to closed position upon resonant operation of at least one of said vibratory elements, a second relay normally maintained in open position, means responsive to resonant operation of another of said vibratory elements only after prior closure of said first relay to actuate said second relay to closed position, a signalling device operable upon closure of said second relay, a signal reproducing means, means controlled by said first relay for controlling reproduction by said reproducing 7: means; and? means: associated with, said; repmduc'-- ing means to maintainqsaid; second; relay: closedupon closure thereof until said reproducing means is -used to receive acall.

In." a. receiving: station adapted to: be callecli from another station, an electromagnet adapted to be energized: by signal frequencies with which, a given carrier is modulated, a. plurality of vibrartory: elements each resonant: at its." respective: signal. frequency, a first: 'relay normally main.- tained in open position, means to more said first: relay. to.- closed. position uponv resonant: operation of at; least one of said vibratory elementsa sec- 0nd: relay normallymaintainecl in, open position-,. means responsive tov resonant operation; of; an:- other of said vibratory elements; only after prior closure of said first relay to. actuate. said second relay-"to closed position, a first signalling device; operable only upon, closure ofboth: relays, a second. signallingdevice operable upon closure: of said second relay, signal reproducing, means, means: controlled by said first relay'for controllingreproduction by said reproducing means, and means associated: With: said reproducing meansto: maintainsaid second relay closed upon closure thereof until said reproducing means: is used to receive'a call.

8'. Ina receiving station adapted to be called from another station, an electromagnet adapted tobe energized by signal frequencies with which a given carrier ismodulated, a plurality of vibra-- tory elements each resonant at its respective signal frequency, a first relay, an oscill'atory'circuit normally operative to maintain said first relay in-open position, means responsive to resonant' operation of at least oneof said vibratory elements to stop oscillations'insaid circuit, thereby enabling said first relay to close, a second relay normally maintained in open position, meansresponsive to resonant operation of another of said vibratory elements only after prior closure of said first relay to actuate said second relay-toclosed position, a, first signalling; device: operableonly upon closure ofbothrelays, and a second signalling device operable uponclosuro ot saidsecondrelay.

9, In a receivingstation adapted to:- be called.: from: another station, anelectromagnet. adapted: tobeenerglzed bysignal frequencies withw which. a given carrier is modulated, a pluralityof vibra tory elements eachresonant: at its respective; signal frequency, a first relay, an oscillatory-cit cuit' normally operative to, maintain saidfirst; relay: in open position, means responsive to, re sonantoperation: of at least one of said vibratory elements to stop oscillations in said circuit, there by-enabling said first relay to, close, a second relay;

REFERENCESICITED lhe following references: are of record inthefile of this patent:-

UNITED STATES PATENTS Number Name Date 1,635,779 Carter July 12,1927 2,131,589 Halligan -.Sept.. 27,1933,

2,325,829 Boswan' Aue. 3,, 194.3 

